The present invention relates to cell containers for use in electrochemical energy storage devices. More particularly, the invention relates to cell caps having pressure release mechanisms that place the cell in open circuit by breaking an electrical contact when the cell internal pressure reaches a defined level.
Due to the increasing demand for portable electronic equipment, there is a corresponding demand for rechargeable cells having high specific energies. In order to meet this demand, various types of rechargeable cells have been developed including improved nickel-cadmium aqueous batteries, various formulations of aqueous nickel metal hydride batteries, and, most recently, nonaqueous rechargeable lithium metal and lithium intercalation cells.
Of particular interest in the context of the present invention are rechargeable lithium-ion cells, although other cell types may benefit as well. Because of the large amounts of energy stored in lithium ion cells and because of the potentially hazardous nature of some cell components, there is a risk of explosion or uncontrolled release of cell electrolyte.
Many lithium-ion cells operate at pressures in the range of 5-25 psi. Such pressures are normally produced by gases generated during the cell's formation cycle and operation Higher pressures, however, can result from overcharge due to a faulty charger, external or internal cell shorting, exposure to excessive heat (e.g., fire), etc. Thus, lithium-ion cell housings should also include some mechanism for controlling the build-up of excess internal pressure. Simply installing a vent in the cell will serve this function by automatically discharging cell contents as internal pressures approach dangerous levels. However, such contents, including flammable organic electrolyte solvents, potentially hazardous electrolyte salts (e.g., lithium-hexafluorophosphate), and even burning lithiated carbon particles should not be released from the cell under pressure unless absolutely necessary to avoid explosion.
Thus, some cells have been designed with safety mechanisms to limit further increases in cell pressure before venting becomes necessary. One representative mechanism is an "18650 cell" (shown in FIGS. 1A and 1B) of manufactured by Sony Corporation of Japan. A similar mechanism is described in U.S. Pat. No. 4,943,497 issued to Oishi et al. As shown in FIGS. 1A and 1B, the top of a cell cap 114 includes a terminal contact 101 for connecting the cell to external circuitry. Of particular relevance here, terminal contact 101 includes vent holes 110, which allow the cell fluid to vent should the pressure in the cell interior become very high. The terminal contact 101 is supported against a positive temperature coefficient resistor ("PTC") 109 which is, in turn, supported against a nipple shaped conductive flexible member 104, with scoring 106. The whole assembly (terminal contact, PTC, and the conductive flexible member) is held together in electrical contact by a first plastic insert 103 and a metal outer jacket 108 as shown. A crimp in the outer jacket 108 gives it a "C" shape which holds the first plastic insert 103 in position against the terminal contact 101 and the conductive flexible member 104.
The nipple portion of conductive flexible member 104 is affixed to a metallic foil 111--which ranges in thickness from about 1-2 mils--by a weld 107. Foil 111 is, in turn, welded onto an aluminum disk 105, about 20 .mu.m thick. As shown in FIG. 1B, the disk 105 includes a central opening, which is covered by foil 111, and three peripheral holes 113 which provide access to the cell interior. Further, peripheral holes 113 are aligned with passages 115 in a second plastic insert 116 so as to provide a fluid pathway from the cell interior to a pressure cavity 102. The second plastic insert 115 electrically insulates the aluminum disk 105 from the metal outer jacket 108, and it is held in place between the aluminum disk 105 and the first plastic insert 103 by the weld 107. Finally, a conductive tab 112 is welded to the bottom of the disk 105 to provide a conductive pathway from the cell cathode to the cell cap subassembly.
During normal operation, current flows from the cathode through tab 112, to disk 105 and foil 111, and then through weld 107 to the assembly of flexible member 104, PTC 109, and terminal contact 101, and finally out to an external circuit. The first plastic insert 103 and the second plastic insert 116 confine the current flow through this defined conductive pathway.
If the cell current reaches unusually high levels, the PTC 109 becomes resistive in response to the high applied current and thereby reduces the current flow. This is because the material used in the PTC typically is a mixture of polymer and carbon. When the current density through the PTC increases to a defined level, the polymer temperature passes a melting transition point and becomes resistive. If the current density drops back below the defined level, the polymer again becomes glassy and the PTC again becomes conductive. In general, this mechanism prevents cell internal heating and dangerous pressure build-up resulting from inadvertent shorting. However, if the problem is unrelated to excessive current flow or if the PTC fails or can not adequately control the current flow, other safety mechanisms are activated.
First, as pressure builds up within the cell, that pressure is transmitted to the pressure cavity 102 where it forces flexible member 104 upward. When the pressure build-up reaches a predefined level, the foil 111 tears at weld 107, thus breaking the conductive pathway to the terminal contact and putting the cell into open circuit. At that point, the cell is electrically isolated from external sources, and no further electrochemical reactions will occur within the cell that could cause the pressure build-up to continue. Although this renders the cell useless as an energy source, the severed electronic pathway hopefully prevents any dangerous consequences resulting from an uncontrolled discharge of cell fluids.
In the event of further excessive pressure build-up due to, for example, extreme temperatures outside the cell, the scoring 106 on the conductive flexible member will rupture to allow venting of cell contents (usually electrolyte) through vent holes 110, thereby preventing an explosion.
While the above design provides some measure of safety, it has certain drawbacks. First, it may be difficult to control the point at which the first safety mechanism places the cell into open circuit. Because the design relies on the breaking of the weld 107 (or tearing the foil 111 around the weld), the strength of the weld may have to meet exacting standards. This, of course, adds to the cell cost. In some cases, it may be possible that the vent design will not perform as intended because the weld 107 or foil 111 resists breaking until the cell pressure is high enough to rupture the scoring 106. Under such circumstances, cell fluids would be expelled before the first safety mechanism could place the cell into open circuit. If, on the other hand, the weld 107 is too weak, the connection 107 may break at slightly elevated, but not dangerous, pressures, resulting in a premature open circuit.
Second, the design may cause the leakage of electrolyte due to perforations in the foil 111. As the cell pressure reaches a predefined level, the foil 111 tears at weld 107. This tearing often creates perforations in foil 111 that may result in premature leakage of electrolyte. In addition, the design is rather complex and includes a large part count.
More recently, a safe cell cap relying on pressure contacts has been described in U.S. patent application Ser. No. 08/509,531 filed on Jul. 31, 1995, naming Mayer et al. as inventors, and entitled OVERCHARGE PROTECTION BATTERY VENT. That application is incorporated herein by reference for all purposes. The cell cap described therein employs a "flip burst disk" which is a dome shaped piece of aluminum metal. In normal cell operation, the dome protrudes from the cell cap, down toward the cell electrodes. In this configuration, the flip burst disk makes a pressure contact (to be contrasted with a welded contact) with a stationary aluminum member and thereby provides an electrical pathway from the cell terminal to the cathode. However, when the pressure within the cell increases to a first critical value, dome inverts (flips) so that it protrudes toward the top of the cell cap. In this configuration, the contact between the flip burst disk and the stationary member breaks and the cell is put into open circuit. If for some reason, the pressure continues to increase to a second critical level, the flip burst will rupture (burst) to release cell contents before an explosion occurs.
While the flip burst disk embodiment described above represents an improvement over conventional designs, it would still be desirable to have alternative designs that do not rely on pressure contacts.